1. Field of the Invention
This invention relates to the production of thin, chemically bonded diamond or diamondlike films. More particularly, it relates to such films produced by ion beam deposition.
2. Description of Related Art
Deposition of diamondlike carbon films has been the subject of intense research for about thirty years. This research has accelerated markedly during the past few years. The basic interest in diamondlike carbon films stems from the unique set of physical properties of diamond: it is the hardest material known; it is an excellent electrical insulator yet is the best thermal conductor known; it has high dielectric strength, and is highly transparent in the ultraviolet, visible and infrared regions of the spectrum; it is chemically inert and therefore resistant to oxidation and corrosion; and, it is biologically compatible with body tissues.
Attempts to fabricate true diamond films have resulted in carbon films having properties which vary over a range of many orders of magnitude. For example, the electrical resistivity of such films has been reported to vary between 10.sup.-2 and 10.sup.12 ohm-cm. The unique characteristics of some of these films and the possibility of "tailoring" a combination of desired properties for a specific purpose result in many advantages of such films for a variety of applications. Proposed applications include: optical coatings (in particular for hazardous environments and outer space); protective thin film coatings for magnetic recording media (e.g., computer disks); heat sinks and high thermal conductivity coatings for semiconductor applications; solid state devices; moisture barriers; low friction coatings for tribological applications; protective coatings compatible with body tissues for medical applications; etc.
The literature on diamondlike films includes several hundred publications, most of them appearing after 1980.
The study of diamondlike carbon films is complicated by the ambiguous and inconsistent nomenclature which has been used in research on these films. These films are referred to as "diamondlike films", "hard carbonaceous films", "hard carbon", "a-C:H", and "i-C". In the past, different names have been used to describe materials which are very similar while at other times the same name was applied to very different materials. This confusion is also related to the sometimes overlooked fact that the field of carbon films covers several pure phases of carbon as well as hydrocarbon compounds. The two best known crystalline phases of carbon are graphite, the stable hexagonal form, and diamond, the metastable cubic form. Diamond is stable at very high temperatures and pressures. In the past two decades three additional metastable carbon phases have been discovered: Lonsdalite (also known as hexagonal diamond); Chaoite (a hexagonal high pressure carbon phase); and, two other cubic, high pressure phases of carbon. Present data on the properties of the carbon phases refers to either cubic diamond or graphite. Very little information is available on the properties of the other phases.
For the purposes of this disclosure, the term "diamond" will be used to refer to a pure carbon material wherein the carbon atoms have sp.sup.3 hybridization. The term "diamondlike" refers to any carbon deposit having a mixture of sp.sup.2 and sp.sup.3 hybridized bonds. The fraction of carbon atoms in a particular hybridization state may vary over a wide range. The process of the present invention may be used to deposit diamondlike (as opposed to diamond) films by employing a C.sup.+ ion beam of very low kinetic energy (less than about 20 eV). Alternatively, the substrate temperature may be adjusted to favor the formation of a diamondlike film. In general, elevated substrate temperatures favor the production of diamondlike rather than diamond films. The particular temperature chosen will depend on the substrate to be coated. For example, at a substrate temperature of 350.degree. C. a low energy, mass-selected C.sup.+ ion beam will not form a diamond film on a nickel substrate (see FIG. 11). In contrast, such a beam will form a diamond film on a gold substrate maintained at 600.degree. C.
The properties of the different phases of carbon appear to be strongly related to the nature of the carbon bond or the electronic structure of the carbon. Cubic diamond has an sp.sup.3 tetrahedral structure wherein each carbon atom is bonded to four different carbon atoms and no "dangling bonds" exist. In contrast, graphite has an sp.sup.2 structure wherein each carbon atom is bonded to only three carbon atoms in a two-dimensional arrangement where the remaining p-type orbital forms a "dangling bond" (or .pi. electron band). "Amorphous carbon" refers to a carbon matrix that includes any possible mixture of sp.sup.1, sp.sup.2, or sp.sup.3 hybridized carbons, and has no crystalline long range order. The term "diamondlike" or "diamond" coating should be reserved for films that possess a true sp.sup.3 electronic configuration. The term "i-C" refers to films prepared using ions. The term "a-C" refers to amorphous carbon while "a-C:H" refers to a hydrocarbon material wherein the hydrogen content varies between about 10% to about 70%. In the latter, the hydrogen-carbon bond can result in an sp.sup.3 structure similar to that of diamond with the exception that a C--H bond terminates the three-dimensional diamond lattice, thus weakening the structure. The marked variation in the properties of different carbon films thus reflects the nature of localized hybridization, which can range from being graphiticlike to diamondlike.
Graphite is the stable phase of carbon under ambient conditions. Deposition of carbon on various surfaces using thermal carbon species thus results in the formation of either graphitic or amorphous carbon films. Such films have a high electrical conductivity and a high absorption coefficient in both the visible and infrared portions of the spectrum. The metastable nature of the diamond phase requires very high pressures and temperatures for formation. Since this pressure/temperature working region is impractical for routine thin film applications, two basic approaches have been adopted for diamond film fabrication:
(a) energetic atomic species (about 10 to 1000 eV) are used for the creation of localized (about 20 .ANG.) high temperature/high pressure regions called "thermal spikes" in the developing carbon layer. The energetic species can be an ionized or neutral atom or a carbon-containing molecule (no additional thermal carbon needed) or any other ion (e.g., Ar.sup.+) that is impinging on the evolving film simultaneously with thermal carbon species: and, PA1 (b) chemical reactions involving hydrocarbons (e.g., methane) and hydrogen at elevated temperatures that result in the formation of diamond layers. PA1 (a) primary ion beam deposition techniques wherein carbon/hydrocarbon atoms are generated, extracted, provided with a controlled amount of energy, and directed onto the substrate. The carbon/hydrocarbon ions can be mass-selected. The carbon ions in these systems are used for both the carbon supply needed for film formation and the energy source for the "thermal spikes" needed for diamond structure formation; PA1 (b) ion beam sputtering deposition techniques wherein an energetic ion beam (usually inert gas ion beam) is directed onto a graphite target and the resulting sputtered carbon atoms and ions deposited on the substrate. The energy distribution of these carbon species depends on the nature of the primary ion beam, the ion energy, and the angle of incidence; and, PA1 (c) dual ion-beam techniques wherein, in addition to the carbon flux of either (a) or (b) above, a second inert reactive ion beam simultaneously impinges on the substrate to be coated. The carbon flux can consist of either energetic or thermal species since "thermal spikes" are generated by the additional ion beam. The complementary ion beam increases the "diamondlike" constituent of the films by preferential sputtering of the graphitic/amorphous carbon regions. PA1 (a) In the initial stage it can force the formation of metastable phases (e.g., of carbidic nature) that can contribute to better adhesion between the film and the substrate. PA1 (b) The ion energy is responsible for the "thermal spikes" in the film that are essential for the formation of metastable carbon phases. A minimal ion energy on the order of several eV may be necessary. PA1 (c) ion energies of several tens of eV and above (depending upon the parent species involved and the angle of incidence) are necessary for preferential sputtering of graphic domains and for increasing the percentage of sp.sup.3 hybridized carbon in the final product. The self-sputtering of the film by energetic carbon ions may be a limiting factor in achieving high deposition rates, for at high ion energies the sputtering rate can exceed the deposition rate. Another beneficial effect of the preferential sputtering is the possible removal of surface impurities. PA1 (d) Ions with energies of several hundreds of eV and above (depending upon the parent species involved) can damage the evolving structure by creating atomic displacements, thereby destroying the sp.sup.3 nature of the deposit. Elevated deposition temperatures (approximately 400.degree.-700.degree. C.) can, however, anneal the damage resulting with diamond formation. PA1 (e) Ions with energies of one keV and above are implanted into the film. Under appropriate conditions that allow annealing of defects, internal growth of diamond occurs. In many practical systems, the energy distribution of the species used for deposition is very broad and uncontrolled, resulting in irreproducibility of the final product.
Tailoring the properties of such films to fit specific applications is sometimes accomplished by annealing the films during or after deposition using laser radiation, energetic ions (about 10 to 1000 keV), etc.
The oldest approach for diamondlike film deposition involves the use of a beam of low energy carbon ions impinging on a substrate surface with a resultant deposition of carbon. Carbon ions and atoms are typically produced by Ar.sup.+ sputtering of carbon electrodes within a magnetically confined plasma operated at a pressure of about 20 to 50 millitorr. The carbon atoms may be further ionized in the same plasma environment. In such systems, the C.sup.+ and Ar.sup.+ ions are introduced into a deposition chamber maintained at about 10.sup.-4 to 10.sup.-6 torr and accelerated towards the sample to energies in the range of about 50 to 100 eV.
Ion beam deposition techniques can be divided into several subcategories:
Different ion beam deposition systems differ markedly in intrinsic position parameters such as the nature and energy distribution of carbon species, beam flux density, ambient pressure during deposition, and composition of non-carbon species in the impinging flux. The significance of some of these deposition parameters for diamondlike film formation is discussed below.
Various plasma deposition techniques have also been used to produce diamondlike films. Plasma decomposition of various hydrocarbon gases results in the deposition of carbon films on substrates placed on a negatively biased electrode. Radio frequency, DC and pulsed plasma systems have been used. Several process parameters related to this technique can be varied and controlled, such as the type of hydrocarbon gas, plasma decomposition power, and substrate bias. Often, an argon/hydrocarbon gas mixture is used, resulting in Ar.sup.+ bombardment of the evolving film that preferentially sputters and partially removes the graphitic and amorphous carbon constituents of the film. A relatively high hydrogen content is typical for these techniques.
Chemical vapor deposition (CVD) processes have also been used to produce diamondlike carbon films. The basic principle of CVD diamond film formation is the use of chemically active hydrocarbon fragments (ions and radicals) for the spontaneous growth of diamondlike material under rather metastable conditions. In a typical experiment, a mixture of methane and hydrogen gas (about 1% methane) is introduced into the system and hydrocarbon fragments, atomic hydrogen, and carbon species are generated using an excitation source (hot filament, radio frequency or microwave plasma). Diamond films are deposited on substrates maintained at a temperature in the range of about 100.degree.-1000.degree. C. Such diamond film formation is strongly dependent on the methane concentration and the substrate temperature. The use of methane is very common, but other hydrocarbons have been successfully used, sometimes resulting in a higher deposition rate. The true crystalline diamond structure of CVD films is now well established.
Discussed below are the parameters related to carbon deposition. This discussion is independent of specific deposition techniques and is presented in terms of the influence of the parameters on the growth and final form of the carbon film.